The invention relates to a corrector for correcting chromatic and spherical aberrations in an electron microscope with six multipole elements disposed consecutively in the beam path such that they are symmetric about a plane of symmetry, of which all are used for generation of quadrupole fields and the third and fourth also for generation of octupole fields, wherein the latter are oriented in the same direction and the quadrupole fields of all six multipole elements are rotated through 90° from one to the next and are centrosymmetric about the point of intersection between the optical axis and the plane of symmetry, wherein the following exchange symmetries are produced: exchange symmetry of the axial fundamental rays with the plane of symmetry as the reflection plane for exchange of the trajectory of the axial fundamental rays, and exchange symmetry of the off-axial fundamental rays with the point of intersection between the optical axis and the plane of symmetry as the point for centrosymmetric exchange of the trajectory of the off-axial fundamental rays, and wherein, thereafter, all fundamental rays are made to converge again at the end of the corrector, wherein correction of the chromatic aberration is possible by the interaction between the magnetic and electric quadrupole fields within the third and fourth multipole element and correction of the spherical aberration is possible by means of the quadrupole fields and the octupole fields.
The purpose of such a corrector is to compensate for the chromatic and spherical aberrations of the optical components of the microscope. However, compensation using such correctors not only compensates for aberrations of the beam source and lenses of the electron microscope but also aberrations caused by the corrector itself.
The basis for the function of all corrections in particle optics is the discovery by O. Scherzer (O. Scherzer: “Sphärische and chromatische Korrektur von Elektronen-Linsen” OPTIK, DE, JENA, 1947, pages 114-132, XP002090897, ISSN: 0863-0259) that correction of chromatic and spherical aberrations is possible for particle beams, too, if non-rotationally symmetrical fields are deployed with which astigmatic intermediate images are generated and this astigmatism is subsequently removed again. O. Scherzer states the conditions under which this can be achieved (Loc. cit.). These conditions known as Scherzer's theorem are the basis for all correction of chromatic and spherical aberrations in particle optics.
From DE 10 2007 049 816 B3, a corrector comprising five multipole elements is known which, however, itself causes not inconsiderable higher-order aberrations, making compensation of aberrations more difficult.
DE 42 04 512 A1 proposed a corrector of the type stated in the introduction that causes fewer higher-order aberrations but is extremely sensitive to field fluctuations of the quadrupole fields. These field fluctuations, also termed “noise,” are due to fluctuations of the supply current and voltage. Sensitivity to these fluctuations proved so great in the test phase of a corrector built according to this reference that it was not possible to control this sensitivity with a highly stabilized electrical power supply with reasonable technical effort. For this reason, implementation of this corrector and the patent application stated above were abandoned.
The object of the invention is to develop further a corrector of the type stated in the introduction in such a way that it is insensitive to fluctuations of the electrical power supply.